1. Field of the Invention
This invention relates generally to rotary cutting tools of cemented carbide such as a drill, an end mill and a reamer and more particularly to such cutting tools in which part of the surface is ground to provide a finish tool.
2. Prior Art
Generally, a drill of cemented carbide with a partially-ground finish surface can be manufactured at lower costs in comparison with a drill of cemented carbide having an entirely-ground finish surface since the former naturally requires less labor and time for the grinding operation. In addition, it is rather difficult to grind such a hard cemented carbide, and therefore the drill with the partially-ground surface is quite advantageous in this respect.
Generally, the cemented carbide of which the drill or the like is made contains cobalt as a binder. In the manufacture of such a drill, when a compacted powder mixture is sintered under vacuum, a sintered surface is formed on the drill body. This surface has a less cobalt content than the remainder of the drill body. The reason is that the cobalt content of the surface is reduced during the sintering. Generally, the drill is grounded at relief surfaces, margins and a shank portion where a sufficient toughness is required for the drilling operation. The remainder of the drill surface, that is, the non-ground portion, is defined by the above-mentioned surface. This surface having a less cobalt content is less tough, and therefore such a partially-ground drill is more liable to damage or breakage in comparison with the entirely-ground drill from which the surface having a less cobalt content is fully removed by grinding. In addition, the non-ground portion of the drill has naturally a greater surface roughness than the ground portion, and this aggravates the problem of breakage of the drill.